The field of the invention is that of the transmission and broadcasting of digital signals, especially in the presence of transmission noises. More specifically, the invention relates to the reception of digital signals encoded by means of a convolutive type error correction code.
The invention can be applied in all cases where digital signals are transmitted or broadcast on noise-infested channels. For example, the present invention can be implemented in receivers of systems for digital radiocommunications with mobile units such as the GSM system.
Other possible applications are, for example, the reception of signals transmitted by RF channels, satellites, etc. More generally, the invention can be applied advantageously in all cases where a convolutive type code is implemented at transmission.
In the known type of receivers, the symbols retrieved at output of the demodulator take the form of analog samples which, after quantification, are generally processed by an equalizer (a transversal filter) given the task of eliminating the inter-symbol interference introduced by the channel. The equalized samples are then de-interleaved if necessary and decoded before being given to the addressee.
The principle of equalization consists in estimating the response of the transmission channel for the application, to the received signal, of a filtering operation symmetrical to this response so as to: obtain a corrected signal. In particular, the equalization is aimed at eliminating or, at least, at limiting the inter-symbol interference introduced by the channel.
The response of the channel is generally estimated by the analysis of a reference signal known to the receivers. Of course, the transmission of these reference signals leads to a reduction of the useful bit rate.
The equalized items of data are then de-interleaved (in an operation symmetrical to the interleaving done at the time of encoding, if such an interleaving is planned) and then decoded.
In certain situations, it happens that the equalization of this type is not sufficient to ensure high quality source decoding.
The article by P. Jung and P. W. Baier, "VLSI implementation of soft output Viterbi equalizers for mobile radio applications" (Proc. of IEEE 42nd Vehicular Technology Society Conference, Denver, Colo., pp. 577-585, May 1992) proposes the use, instead of a transversal filter, of a symbol detector working according to the principle of maximum likelihood.
This technique is more efficient if the coefficients representing the channel are properly estimated. However, once again, the equalized items of data may prove to be of insufficient quality. Furthermore, it is more complicated to make and requires more space than with the transverse filter technique.
Convolutive codes are codes that associate at least one encoded item of data with each item of source data to be encoded. This encoded item of data is obtained by the modulo 2 summation of this item of source data with at least one of the previous items of source data. Thus, each encoded signal is a linear combination of the item of source data to be encoded and previous items of source data taken into account.
In the decoder, the original items of data are most usually reconstructed by means of a maximum likelihood algorithm, for example the Viterbi algorithm, whose decisions may be weighted if necessary. The Viterbi algorithm, in taking account of a sequence of received encoded symbols, provides an estimation of each item of data encoded at transmission, in defining the source sequence most probably corresponding to the received sequence.
The Viterbi algorithm may also be used to detect sequences affected by inter-symbol interference. The invention can be also applied in this case.
Clearly, the greater the number of symbols taken into account, the more reliable is the decision. By contrast, the greater this number, the more complicated is the decoder or the detector (hereinafter, the term decoder is used to describe decoders themselves as well as detectors. This observation is applicable also to the term decoding which must be understood to mean decoding or detection as the case may be. The memory space needed soon becomes very great, as do the corresponding computation times.
The integrated circuits that implement such algorithms therefore most usually rely on a compromise between cost and performance. These industrial-level choices do not always enable the construction of decoders that correspond in an optimum way to a given application. For example, it is not possible to make low-cost decoders for applications where the reception quality is not of crucial importance, as integrated circuits cost too much. Conversely, these integrated circuits too are not suited to the making of receivers with very high decoding quality for which the cost price is of little importance.
An advantageous example of convolutive encoding, to which the invention can be applied, is described in the French patent application FR 91 05280 filed on behalf of the same Applicants as well as in the article by C. Berrou, A. Glavieux and P. Thitimajshima, "Near Shannon limit error-correcting coding and decoding: turbo-codes" (Proc. ICC'93, pp. 1064-1070, Geneva, Switzerland, May 1993). This class of codes is known in particular as "turbo-codes".
The elementary codes (recursive systematic or pseudo-systematic codes) described in the French patent application FR 91 05278 filed on behalf of the present Applicants may also be used.
The invention is aimed especially at overcoming the drawbacks of the prior art reception device and more specifically at improving the corresponding performance characteristics.
More specifically, a goal of the invention is to provide a device of this kind with very high corrective capacity as compared with known methods presently used in digital communications systems.
The invention is aimed in particular at providing methods of this kind that are particularly efficient, again with respect to known methods, for transmission in highly noise-infested channels.
It is also an aim of the invention to provide a device of this kind that is highly efficient but nevertheless easy to manufacture on an industrial scale at acceptable costs.
Thus, a particular aim of the invention is to provide a decoding method enabling implantation on a silicon surface that is small enough for its industrial-scale manufacture to be possible, for example, on a surface area smaller than 50 mm.sup.2.
It is also an aim of the invention to provide a method of reception used for the making of numerous types of receivers with performance characteristics and cost price that vary as a function of the needs fulfilled, implementing one or more integrated circuits of a single type.
In other words, an essential aim of the invention is to provide methods of this kind enabling firstly profitable industrial-scale manufacture based on the development of a single and relatively simple integrated circuit and, secondly, the making of receivers that can be used for a very wide variety of applications.